I have a Array of constant size defined like this

Var Constant
MAX_SIZE :UINT := 10;
End_var

VAR
array : ARRAY[0..MAX_SIZE] OF INT;
END_VAR

Now I want to pass this array to a function by reference,

Function fun : INT
VAR_IN_OUT CONSTANT
MAX_SIZE : UINT;
END_VAR
VAR_INPUT
array : REFERENCE TO ARRAY[0..MAX_SIZE] OF INT;
END_VAR

but when I run this it gives strange Errors

Error : Cannot Convert type 'ARRAY [0..MAX_SIZE] OF INT' to type 'ARRAY[0..MAX_SIZE] OF INT'

Could not find the solution to this issue but I have side stepped this by creating a global variable MAX_SIZE.

maybe this would be a better approach, then you don't have to pass the constant at all.

FUNCTION fun : INT
VAR_IN_OUT
    arr:        ARRAY[*] OF INT;
END_VAR
VAR
    lower:      DINT;
    upper:      DINT;
END_VAR

lower:= LOWER_BOUND(arr, 1);
upper:= UPPER_BOUND(arr, 1);

see here as well
https://help.codesys.com/api-content/2/codesys/3.5.12.0/en/_cds_datatype_array/

It will be a reference, because it's a VAR_IN_OUT.

it's a little weird debugging the array online. It just shows as a POINTER TO INT, and you can't actually see the array from the function side. But you can still work with it as a normal array, not a pointer.